It is well known that a silver salt photograph generally has high image gloss, is excellent in image gloss uniformity and color reproducibility, and is excellent in image quality. In addition, the silver salt photograph is characterized by high productivity, and can be printed in a large number per unit time.
Meanwhile, image quality has been steadily increasing in electrophotography owing to digitalization in recent years. However, compatibility between high image gloss uniformity and high productivity has not been achieved yet, so there has been desired a full-color image forming method according to electrophotography in which compatibility is achieved between high image gloss and high productivity. In addition, there has also been desired an image forming method, with which images without a sense of incongruity each of which has image gloss uniformity comparable to that of a silver salt image, and has a balance between image gloss and recording medium gloss can be stably and continuously obtained, at a relatively high recording medium conveying speed while high image gloss and high productivity are made compatible.
As a full-color image forming method according to electrophotography achieving compatibility between high image gloss and high productivity, a high-speed machine which has multiple developing units arranged in series and can output a monochrome image and a full-color image at the same speed has been put on the market. However, when an image having high image gloss is desired, for example, a recording medium conveying speed is reduced to increase image gloss. As a result, productivity has been reduced.
In general, when attention is paid to a fixing step of a copying machine having a relatively high recording medium conveying speed, the time necessary for a recording medium to pass through a fixing unit is relatively short, so a heat quantity to be applied to the recording medium reduces. A reduction in heat quantity to be applied to the recording medium directly leads to a reduction in image gloss. Therefore, an increased heat quantity must be applied in order to obtain an image having high gloss without impairing productivity.
A method of obtaining an image having high image gloss at a relatively high recording medium conveying speed involves: widening a fixation nip; and applying an increased heat quantity to a recording medium. For example, in a roller fixing unit referred to as a roller nip type shown in FIG. 1, fixing members are brought into strong abutment with each other to widen a fixation nip.
In addition, there is a belt nip type fixing device shown in FIG. 2 in which a fixing roller and a conveying and fixing belt are combined ((FIG. 2) shows a technique in which the conveying and fixing belt is combined to widen a nip between the belt and the fixing roller to cope with an increased speed).
Based on the same concept that an increased heat quantity is applied for obtaining an image having high image gloss at a relatively high recording medium conveying speed, there is a device having multiple fixing units arranged as shown in FIG. 3.
Such a device is described in, for example, JP-A 2000-221821 (Patent Document 1). The document describes a method involving: performing fixation by using multiple fixing units arranged in series in a recording medium conveying direction while switching the number of fixing units to be used; and changing a heat quantity to be applied to a recording medium to arbitrarily adjust the gloss of a printed image. With the method, image gloss is adjusted without a reduction in printing speed. However, the document has no description relating to toner physical properties, and has no discussion about the relationship between any one of the toner physical properties and gloss.
A conventionally known example referring to: a fixing device having multiple fixing means; fixing conditions of multiple fixing units; and toner properties includes JP-A 04-287078 (Patent Document 2). The document has proposals regarding a toner softening point and a fixing temperature in a method involving heat-pressure-fixing color toner onto a transparent recording medium, in particular, a heat-pressure-fixing method involving: performing temporary fixation in first fixation; and performing actual fixation in second fixation. However, in the document, recording medium is limited to a transparent recording medium such as an over-head projector sheet, and recording medium conveying speeds in the first fixation and the second fixation is limited to relatively low recording medium conveying speeds, so it does not assume the use in a high-speed machine. Therefore, there remains, for example, a problem in that sufficient transparency cannot be obtained at a relatively high recording medium conveying speed. In addition, the document refers to the set temperature of a fixing unit, but the temperature on a recording medium is not directly measured in the document. In the case of fixation to a recording medium such as paper unlike a transparent medium such as an over-head projector sheet, molten toner flows into a concave portion formed by a fiber of the recording medium, so the image surface becomes uneven and image gloss reduces in some cases. In addition, there is, for example, a problem in that fixation strength becomes insufficient, and color-mixing ability becomes insufficient to result in poor color reproducibility.
As described above, investigations have been made with a view to achieving compatibility between high gloss and high productivity. However, there has been nearly no investigation in which attention is paid to the relationship with any toner physical property. Moreover, no research has been made on a method of obtaining an image without a sense of incongruity which has high gloss uniformity, and has a small difference between recording medium gloss and image gloss at a relatively high recording medium conveying speed.
Patent Document 1: JP-A 2000-221821
Patent Document 2: JP-A 04-287078